Engineers typically design high-pressure oil field plunger pumps in two sections; the (proximal) power section (herein “power end”) and the (distal) fluid section (herein “fluid end”). The power end usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. Commonly used fluid ends typically comprise a fluid end housing having one or more sub-assemblies, each sub-assembly comprising a central cavity, a suction valve in a suction bore, a discharge valve in a discharge bore, a plunger in a plunger bore, and an access bore plug in an access bore, plus retainers and high-pressure seals (including plunger packing), etc.
FIG. 1 shows a cross-sectional schematic view of such a typical fluid end sub-assembly showing its connection to a power end by stay rods. A plurality of fluid end sub-assemblies similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid end housing design schematically illustrated in FIG. 2.
Components internal to the fluid end housing typically include a suction valve for controlling fluid flow in the suction bore, a discharge valve for controlling fluid flow in the discharge bore, and an access bore plug for reversibly sealing access to the central cavity via the access bore. Note that the terminology applied to fluid end sub-assembly suction and discharge valves varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the moving element or valve body, whereas the term “valve” as used typically herein includes the valve body, the valve seat, one or more valve guides to control the motion of a valve body, and one or more valve springs that tend to hold a valve closed (i.e., with the valve body reversibly sealed against the valve seat), plus spring retainers, spacers, etc.
Fluid end housings are subject to catastrophic failure (due, for example, to severe over-pressure caused by an obstruction in the fluid discharge path), as well as fatigue failure associated with peaks of cyclic stress resulting from alternating high and low pressures which occur with each stroke of a plunger cycle. Local maxima of peak cyclic stress are concentrated near various structural features of a fluid end housing. Catastrophic failures are relatively infrequent but fluid end housings fail more commonly in areas of cyclic stress concentration where fatigue is greatest. For example, fatigue cracks may develop in one or more of the areas defined by the intersections of the suction, plunger, access and discharge bores with the central cavity as schematically illustrated in the (generally right-angular) bore intersections schematically illustrated in FIG. 3.
To reduce the likelihood of fatigue cracking in fluid end housings, a Y-block housing design has been proposed. The Y-block design, which is schematically illustrated in FIGS. 4 and 5, reduces stress concentrations in a fluid end housing such as that shown in FIG. 3 by increasing the angles of bore intersections above 90°. In the illustrated example of FIG. 4, the bore intersection angles are approximately 120°. A more complete cross-sectional view of a Y-block fluid end sub-assembly is schematically illustrated in FIG. 5. Note the absence of an access bore as shown in FIGS. 1 and 3.
Although several variations of the Y-block design have been evaluated for field use, none have become commercially successful for several reasons. One reason is that mechanics find field maintenance on Y-block fluid ends relatively difficult. For example, the absence of an access bore makes replacement of plungers and/or plunger packing significantly more complicated in Y-block designs than in the design shown in FIG. 1. Access to both a plunger and its packing in a fluid end sub-assembly like that of FIG. 1 is conveniently achieved by pushing the plunger distally through the plunger bore and out through the access bore, followed by removal of the packing proximally. This operation, which leaves the plunger packing easily accessible from the proximal end of the plunger bore, is impossible in a Y-block design. And since a plunger must fit very tightly within its packing, removal of the plunger packing with the plunger in place (as seen, for example, in FIG. 6) is very difficult in the field. Thus, notwithstanding their nominally higher resistance to fatigue failures at bore intersections, Y-block fluid ends have rarely been used when a fluid end similar to the design shown in FIG. 1 is available.
A brief review of plunger packing design will illustrate some of the problems associated with packing and plunger field maintenance in Y-block fluid ends. FIG. 6 schematically illustrates an enlarged view of the packing in an earlier (but still currently used) fluid end sub-assembly such as that shown in FIG. 1. In FIG. 6, the packing and packing brass are shown installed in the packing box of the fluid end sub-assembly. Note that “packing brass” is a term used by field mechanics to describe bearing bronze, where the bronze has the appearance of brass.
In the fluid end sub-assembly portion schematically illustrated in FIG. 6, the packing box is an integral part of the fluid end housing; it may also be a separate unit bolted to the housing. The packing is retained by the gland nut, and the tightness of the packing about the plunger may be increased by turning the gland nut. Loosening or removing the gland nut, however, does little to release the tight fit of the packing rings on the plunger. Since the packing rings must block high-pressure fluid leakage past the plunger they are typically quite stiff, and they remain substantially inaccessible in the packing box while the plunger (or any piece of it) remains in the plunger bore. FIG. 7 schematically illustrates such a situation, with the gland nut removed from the packing box and the distal end of the plunger (i.e., the pressure end) remaining within the box. Note that even though the plunger is shown disconnected from the crosshead extension rod, the plunger pressure end still cannot be rotated for removal until it has been withdrawn sufficiently to completely clear the packing brass. In view of the limited space between the power and fluid ends, withdrawal of the plunger is facilitated if it comprises two or more pieces reversibly connected together. But the advantage of being able to deal with two relatively short plunger pieces is somewhat offset by the necessity for disconnecting and reconnecting the pieces when replacing or otherwise servicing the plunger packing.
The field maintenance problems associated with multi-piece plungers in Y-block fluid end housings have not been eliminated by the recent introduction of packing assemblies such as those called “cartridge packing” by UTEX Industries in Houston, Tex. An example of such cartridge packing is schematically illustrated in FIG. 8. Note that removal of the gland nut exposes the packing cartridge housing, which in turn may be fitted with attachment means to allow extraction of the packing cartridge from the packing box (commonly requiring proximal travel of the packing cartridge housing of approximately three to five inches).
Even with use of the above attachment means however, extraction of the packing cartridge is not practical while a plunger piece lies within the packing box. This is because of the substantial drag force of the compressed packing rings on the plunger and packing box walls. Unfortunately, the drag force can not be reduced unless all plunger pieces are removed from the packing box so as to release the compression of the packing rings. Further, any slight misalignment of the attachment means and/or the apparatus used to extract such a packing cartridge assembly tends to cause binding of the (right cylindrical, i.e., not tapered) cartridge within the (right cylindrical) bore in which it is installed. Analogous difficulties occur if an attempt is made to replace such a cartridge packing assembly while a plunger or part thereof lies in the packing box area. Hence, even if such cartridge packing assemblies were used in Y-block fluid section housings with multi-piece plungers, field maintenance would still be relatively complicated and expensive.
Thus, although the Y-block fluid end housing is characterized by a generally lower likelihood of fatigue failure than earlier right-angular fluid end housing designs, it is also associated with significant operational disadvantages. Improved fluid ends would offer weight reduction, easier internal access for maintenance, and/or reduced likelihood of catastrophic and/or fatigue failures.